In such boost converters that operate in voltage mode at a fixed frequency, the output voltage is a function of the duty cycle which depends on the ratio between the on-time and the off-time of the switch. As long as the load current is high enough, the converter can operate in continuous mode: the inductor current increases when the switch is closed, and decreases when the switch is open, but it will never become zero. In this mode, the output voltage is a function of the duty cycle only and does not depend much on the output current.
If the load current decreases below a lower limit, the inductor current will cease during part of the off-time, and the converter works in discontinuous mode. In this mode, the relation between duty cycle and output voltage is different, so the control of the switch has to be adapted, e.g. the control for the switch may turn to a different loop gain or into a power save mode. Therefore, it is necessary to detect the transition from continuous to discontinuous mode and back by monitoring the coil current. Usually, this is done by monitoring the output current, for example by measuring the voltage drop across a sensing resistor in the output path or across the rectifying device during the off time of the switch.
FIG. 1 shows an example of such a conventional asynchronous boost converter. The converter has a supply voltage input 10 and a boosted voltage output 12 and comprises a serial circuit of an inductor 14 and a diode 16 between the supply voltage input 10 and the voltage output 12. The connection between the inductor 14 and the diode 16 constitutes a connection node 20. A transistor 24 acts as a switch, connected between the connection node 20 and ground and is controlled by a control circuit 26. A comparator circuit 28 for monitoring the inductor current in the off-phase has one input connected to the connection node 20, another input connected to the voltage output 12 and an output connected to the control circuit 26. The comparator 28 is only enabled and evaluated during the off-phase of the transistor 24. In this phase, the comparator 28 senses the voltage drop over the diode 16 and provides an output signal for the control circuit 26 when the inductor current becomes zero.
The diagrams of FIG. 2 show the inductor current IL, the corresponding voltage Un at the connection node 20 and the comparator output signal COMP over time. As long as the inductor current does not reach zero, the converter operates in continuous mode. The voltage Un at the node 20 changes between nearly zero in on-phase and the output voltage plus the diode voltage. This mode comprises the first four periods in FIG. 2.
When at t=t0 the inductor current becomes zero, the voltage at the node 20 drops to the level of the input voltage Uin and the converter starts operating in the discontinuous mode. Parasitic inductance and capacitance at the node 20, e.g. due to the FET-transistor 24 and the length of the connection lines to the diode 16, limit the decay speed of the node voltage Un. Therefore, it may take a few ten of ns until the voltage Un crosses the level of the output voltage Uout, which means that the voltage drop over the diode and hence the inductor current have become zero, and the comparator 28 can detect the transition.
In FIG. 2, it is not until the second instance t02 of the inductor current IL reaching zero, that the comparator delivers an output signal. Since the comparator 28 also adds some nanoseconds for detection, a delay δ as indicated in FIG. 2 occurs between the rising edge of comparator signal and the time when the inductor current actually gets zero. At high switching frequencies (>1 MHz) the impact of this delay δ gets too high and results for example in efficiency loss, if the comparator signal is used to switch into a power save mode. Further, especially when high output current is required, the diode is preferably located outside the chip hosting the other components, resulting in high parasitic inductances caused by bond wires.